There may be many reasons. I think succinctly,
This is about supporting a program for all of the world. They have been talking about that for many years.
The financial support alone is skewed by our donations to the BSA.
Also, BSA skills/results may not be furthering the missions of the church as much as they think they can do with our own programs.
1. Preach the Gospel
2. Redeem the Dead
3. Perfect the Saints
4. Care for the Poor and Needy